1. Field of The Invention
This invention relates to a wireless communications and, more particularly, to a multiple branch receiver architecture in a wireless communications system.
2. Description of Related Art
The service area of a wireless communications system is partitioned into connected service domains known as cells, where wireless units communicate via radio links with a base station (BS) serving the cell. The base station is coupled to a land network, for example through a Mobile Switching Center (MSC) which is connected to a plurality of base stations dispersed throughout the service area. In the wireless communications industry, a service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an xe2x80x9cAxe2x80x9d band provider for cellular communications receives frequency channels within the A (825-835 MHz), Axe2x80x2(845-846.5 MHz) and Axe2x80x3(824-825 MHz) bands, and the wireless units receive frequency channels within the A (870-880 MHz), Axe2x80x2(890-891.5 MHz) and Axe2x80x3(869-870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835-845 MHz) and Bxe2x80x2(846.5-849 MHz) frequency bands, and the wireless units receive frequency channels within the B (880-890 MHz) and Bxe2x80x2(891.5-894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more PCS bands (1850 MHz-1910 MHz), and the wireless units receive frequency channels on one or more PCS bands (1930-1990 MHz).
In order to reduce system hardware costs, a service provider would want to use a common receiver for the simultaneous reception and processing of signals within the non-contiguous frequency bands. In a typical receiver architecture, a down-conversion stage for each frequency band is typically used to down-convert and to manipulate the placement of each frequency band at intermediate frequencies (IF) such that the frequency bands of the modulated analog signals are converted to a corresponding IF frequency spectrum and can be sampled at a reduced sampling rate by separate analog to digital (A/D) converters. To use a single A/D converter to digitize the modulated analog signals in the non-contiguous bands, a single A/D would have to sample at a high enough rate to encompass both frequency bands. This is an inefficient approach because the A/D converter is using bandwidth in sampling unwanted frequencies in the gap between the frequency bands. To reduce the frequency gap between non-contiguous frequency bands, a down-conversion stage for each of the frequency bands is used to down-convert and manipulate the placement of each frequency band at IF such that the bands are closer together to fit in a smaller bandwidth for the A/D converter. Another approach to improve the efficient use of the A/D converter bandwidth involves down-converting both frequency bands such that a replica of one of the frequency bands is positioned in the frequency gap between the frequency bands.
When the IF spectrum is sampled by an A/D converter at a sampling rate which is greater than or equal to twice the combined signal bandwidth, which can be referred to a the Nyquist sampling rate, the A/D input signal bandwidth rotates or folds periodically about itself at multiples of one-half the sampling frequency. As such, the signal bandwidth and mirror images of the signal bandwidth are periodically repeated at frequency intervals corresponding to the sampling rate of the A/D converter. Each replica of the signal bandwidth can be referred to as a Nyquist zone, and the IF signal bandwidth folds back to the first Nyquist zone between about 0 Hz and one-half the sampling frequency. The bandwidth of a Nyquist zone corresponds to the Nyquist bandwidth.
The periodicity of the spectral density in the digital domain is a basic property of sampled waveforms which can be predicted by determining the Fourier transform of the time-sampled waveform. Generally, the A/D converter samples at at least twice the signal bandwidth of the composite frequency bands (i.e. the Nyquist sampling rate) to obtain a digital representation of the modulated analog IF signal. Accordingly, the sampling rate for the A/D converter is chosen such that the Nyquist bandwidth encompasses the desired frequency bands. The higher the sampling rate, the wider is the Nyquist bandwidth. If the waveform is sampled at a rate less than twice its signal bandwidth (the Nyquist bandwidth), an undesirable overlapping between the adjacent periodic spectrums occursxe2x80x94a well known phenomena known as aliasing. Accordingly, the sampling rate and the IF frequency are chosen such that the Nyquist bandwidth encompasses the frequency band to be converted while reducing the sampling rate of the A/D converter, enabling the use of lower sampling rate A/D converters with reduced cost. Accordingly, the wider the separation or frequency gap between the frequency bands, the current receiver architectures reach a point where the use of a single A/D is not viewed as practical or efficient. If the frequency bands are far enough apart or if desired, a separate antenna is used for each segregated frequency band. In multiple antenna architectures where antennas are dedicated to different frequency bands, a separate A/D is typically used for each antenna path.
Wireless communication base stations also use multiple antennas receiving the same frequency band to support a technique known as N-way receive diversity to mitigate the effects of multipath fading. The base station comprises one or more radios that comprises N spatially-separate receive antennas (xe2x80x9cRxc3x971xe2x80x9d through xe2x80x9cRxc3x97Nxe2x80x9d). Because multipath fading is a localized phenomenon, it is highly unlikely that all of the spatially-separated receive antennas will experience multipath fading at the same time. Therefore, if an incoming signal is weak at one receive antenna, it is likely to be satisfactory at one of the others. For example, when the topography of the terrain is hilly or mountainous, or when objects such as buildings or trees are present, a signal transmitted by a wireless unit can be absorbed or reflected such that the signal quality is not uniform at the base station. As such, many independent paths result from the scattering and reflection of a signal between the many objects that lie between and around the wireless unit and the base station. The scattering and reflection of the signal creates many different xe2x80x9ccopiesxe2x80x9d of the transmitted signal (xe2x80x9cmultipath signalsxe2x80x9d) arriving at the receive antenna of the base station with various amounts of time delay, phase shift and attenuation. As a result, the signal received at the base station from the wireless unit is made up of the sum of many signals, each traveling over a separate path. As the multipath signals are added constructively and destructively at the receive antenna of the base station, severe local variations in the received signal strength can occur. This phenomenon is widely known as multipath fading or fast fading or Rayleigh fading.
As is well-known in the prior art, a diversity combiner can combine N incoming signals, each from one of N receive antennas, using various techniques (e.g., selection diversity, equal gain combining diversity, maximum ratio combining diversity, etc.) to reduce the adverse effects of multipath fading and improve the reception of an incoming signal. In diversity combining techniques performed in the digital domain, the incoming analog signals from the N receive antennas are maintained on separate channel branches and provided to separate analog to digital (A/D) converters on each channel branch for conversion in the digital domain where diversity techniques can be used to improve reception of the incoming signal. Using multiple A/D converters increases costs and can result in reduced performance due to an incoherence between the time samples performed by separate A/D converters of the analog signals from the N receive antennas. Removing any incoherence between the time samples of the incoming signals from the N receive antennas is important when accurate measurements of time delay or phase shift is required. Alternatively, the incoming analog signals from the N receive antennas can be combined or selected prior to digital conversion according to a diversity technique performed in the analog domain, and the resulting analog signal is provided to a single analog to digital (A/D) converter for conversion to the digital domain.
The above multiple branch receiver architectures do not take advantage of the potential bandwidths, flexibility and/or time and/or phase coherence capability provided by A/D converters in converting analog signals into the digital domain.
The present invention involves a receiver which provides received analog signals to a plurality of channel branches, and on at least one of the channel branches, the frequency of the received analog signals is adjusted independent of the relative positions of the corresponding analog signals in the radio frequency (RF) spectrum. The analog signals on the channel branches are then combined, and the combined analog signals are converted into the digital domain. For example, the receiver comprises at least one antenna(s) which receives radio frequency (RF) analog signals. A channel branch arranger receives the analog RF signals from the antenna(s) and provides the RF analog signals to a plurality of channel branches. A frequency conversion arrangement comprising at least one frequency converter on at least a respective one of the channel branches adjusts the frequency band of the analog RF signals on the respective channel branch independent of the relative positions of the corresponding analog signals in the RF spectrum of the different channel branches. The analog signals on the channel branches are combined, and a single analog to digital converter converts the combined analog signal into digital signals. In converting the composite analog signals into the digital domain, the frequency bands of the analog signals are positioned in a plurality of Nyquist zone channels in the digital domain. By properly selecting the frequency bands for the analog signals on the channel branches and the sampling rate for the A/D converter, the available bandwidth for the A/D converter can be more efficiently used, and/or time coherence and/or phase coherence can be provided.